Process for the recovery of sulphur dioxide from blow-pit gases



sept. 14, 1926. 1,599,490

G. A. RICHTER ET AL PROCESSOB THE RECOVERYy OF SULPHU DIOXIDE FROM BLOW PIT GASES Filed April 22 j 1925 w IH mh:

@am m UZ@ 71 @529m Patented Sept. 14, l192.6.

f UNITED STATES PATENT OFFICE.l f

GEORGE A. .RICHTER AND WALLACE B. VAN ARSDEL, OF BERLIN, NEW HAMPSHIRE,

ASSIGNORS TO BROWN COMPANY, OF B OF MAINE. v

ERLIN, NEW HAMPSHIRE, A. CORPORATION PnooEss non THE RECOVERY or sULPHUR Dioxins4 FROM snow-rrr GASES.

Application led April 22, 1925. Serial No. 24,969. A

the blow pit during the blowing operation of tower, in such manner that the major portion of the steam and .vapor is condensed, the remaining gases being delivered in a relatively cool condition to an absorption system. (See U. S. Letters Patent to George 1A, Richter, No. 1,469,958, October 9 1923.

The eiconomical operation of such a systeniv must be carefully balanced between the volume of cooling water supplied to the' cooling tower or condenser and the volume of vapors and gases liberated in the blow pit. If too large a Q quantity of coolingv water is used in the tower, the effluent water will be relatively cool, and anJ excessive amount of dissolved sulphur dioxide will be carried away therewith; if too small a quantity of' water is employed, it will not y have entirely condensed out the. steam' or sufciently diminished the heat content of the gases, -and consequently they will emerge hot and moist from the tower, rendering inellicient the operation of the absorptlon System.v The tendency in the sulphite mill is always to avoid Athe latter ev1l, so that 1t is usuallythe case that a large quantity of water is used, resulting in'a cool eliluent which carries away large quantities of disf` solved sulphurdioxide which 1s lost, or

. which re uiresa separa-te means to recover the ab'sor ed gas from the condensing water. Recovery of thetsulphur dioxide from the eluent -is feasible andv may be resorted to, but it emay not always be economical. Moreover, an indirect method of cooling the vapors and. gases byv a surface cooler orv condenser of the tubular type is practically out -of the question, because it would in-'v volve a large sized apparatuswith large in itial and maintenance cost. 4 -The object Vof thls invention 1s .to provide an economical and eiiicient method ofrecovery of sulphur dioxide from the vapors and gases liberated in the ,blow pit during a blow `of a sulphite digesten This object is atr tained' by employing a small quantity of cooling water in the cooling tower or con? denser, thereby abstractingv 'as large aportion of the heat content of the gases and` condensing as large a portion of the steam rising from the blow pit' as is possible with substantially no sulphurdioxide loss due to absorption by the cooling water, and then passing the partially cooled gases and vapors through a second cooling tower or condenser filled with cold, inert,interstitial material, thereby further diminishing the lheat content of the gases to a point where efficient operation of the absorption system results. j Y

The accompanying drawing illustrates, more or less conventionally and diagrammatcally, an apparatus by which the process may be carried out.

The contents of a digester 10 of any type suitable for the digestion of cellulosic material under heat and pressure with a sulphite liquor, as for example a calcium bisulpli-ite liquor, are discharged into a blow pit 5 by a blow pipe 3 valved, as at 2, the digester coitentsgbeing blown at the usual target 6 in the pit.' Above the pit and communicating therewith through its top portion adjacent to the side opposite to the target side, is a conduit having a short vertical section 13 and a horizontal section 14,

which is supported by a perforated horizpn- I i i tal partition 2() disposed a short dist/ance above the top of the horizontal section/14 of the conduit. Cold water may be pumped ,'from a supply source 21v through a pipe 23 .f by'a pump 22 and distributed more or less uniformly over ,the'top of-.the surface material 15' by a, spray or distributor, as at 24. The water isthus distributed over and wets the downwardly percolating, relatively cold water, a large portion of the steam being condensed and removed therefrom, and are dra-wn out of the top of the condenser tower through a pipe 27. The water is heated on its downward passage through the tower by the hotl gases and forms, together with-the condensate fromthe steam which it carries ltherewith, a pond at the bottom 16 ofthe tower, with which the hot gases contact as rising above the fixed level in the they pass through the apertures 19,'this acting to raise the temperature of the water in the vpond and to liberate dissolved gases therefrom, and somewhat lower the temperature of the hot gases contacting therewith. A short distance above the bottom 16 of the tower. and connected to its side, is a constant level pipe 40 with a trap 37 interposed therein, which pipe may, if desired, discharge into the pit. dt Amaintains a lixed liquid level in the pond so that the water pond is continuously discharged by gravity. If this water' is conducted to the blow pit, it is preferably discharged at a point so that it impinges against the hot digester contents as they are being blown in a, stream against the target 6, thereby further raising the temperature of the waterv and tending to liberate any last traces of dissolved gases contained therein. The liquid contained in the trap 37 prevents any gasesV from the blow pit from entering into the tower througl the pipe 40. Suitably connected to the gas pipe 27 and the constant level water pipe 40 are the recording thermometers 38, 38, so that the temperature of 'the water leaving the bottom of the tower and the temperature of the gases leaving the top of the tower may be ascertained at any time during the blow. The flow of water into the condenser tower may thus be suitably regulated so as to maintain the desiredtemperature of eiiiuent water and cooled gases. Ordinarily, in the practice of our process, the efliuent waterjfrom the condenser is at a temperature of -approximately 200 F., and. consequently at thi's temperature the solubility `of SO2 therein is so low that no appreciable proportion of SO2 is carried by the water into the' blow pit. Hence, if thel eiliuent water is delivered to the blow pit, it serves to wash the pulp with a stream of hot water. The eiiituent may be discharged into a waste pipe external to the blow pit, and employed for any other suitable purpose. The partially cooled gases emerging from the top of the tower through the pipe 27 are passed to the bottom of a second cooling tower 35 of about the same size as the tower 18, which is also filled with cold, inert interstitial material 32 supported on a perforated partition 36.

The gases rise through the surface material filled tower, give up a further portion of' their heat content to the cold inert' material and are drawn 'out through the pipe 41, valved at 43. As the blow progresses, the cold material rises in temperature, thetemperature of the gases emerging from the top of the tower also rising. During the time interval between blows, the` interstitial material is cooled down to its initial temperature by water supplied from .the pipe 4:0, which is suitably sprayed or distributed as by a .distributor 43, uniformly on the top of the material, the cooling water being run to waste from the bottom of the tower by a waste pipe line 39 or conducted to some point 1n the plant where its acquired heat content may be advantageously utilized. In some instances, the condensate from pipe 39 may be 4conducted to some convenient point in the system by which raw acid liquor is produced.

The cooled gases emerging from the top of the tower 35 through a pipe 41,- now contain a relatively small percent of water vapor together with larger percentages of' sulphur dioxide and inert gases, and are passed into the bottom portion'of the recovery or absorption tower 50 filled with crushed -inert mineral material 52. Cold water may be run in atthe` top of the absorpt'iofn tower at the proper rate through a pipe 51', slowly trickling down through the mass of material and absorbing the soluble sulphur dioxide, so that an'acid liquor is dischargedat the bottom of the tower 50 through a pipe 59; any' unabsorbed or undissolved :inert gas being withdrawn from the top of the tower 50 by a pipe 60 con-l nected to a vacuum pump 34 which creates -a partial vacuum in the entire system-behind the blowpit and maintains the gas Aliow through the system. Preferably, hour` ever, there is introduced into tower 50 through pipe 51 what may be termed a cold,

raw acid liquor, e: g., a calcium sulphite liquor, produced by causing sulphur burner contain about 15%-2'0% SO2, would Howto the raw acidliquor absorption system.

A specific example of the foregoing proc- 120 gases and water to flow, counter to each" ess may be had from the following surnmary. After the cooking of wood chips in the digester ina suitable sulphite liquor has been effected, the exhaust pump 34 is set' run into the tower 18 at the required rate,

so that the lar e quantities of steam and gas, which are lierated during the blow and which rise through the conduit sections 13 and 14 and pass 4down through the apertures 19 of the section 14, come into contact with the water pond 16 at the bottom of the tower. The gases, 90% by volumje of which is steam/ and 10% permanent gases, principally nitrogen and sulphur dioxide, are at 'a temperature of about 210-212 F. (depending on the barometric pressure), so that the water in the pond, which is at a lower temperature, partially cools the1hot gases', any dissolved gases in the pond water being driven therefrom as this water is" heated, and swept up with the 'steam and other gases. The gases then pass up through the materi l filled tower and come into contact with the multitudinous streamsofl watertrickling counter-current thereto, alarge proportion of the steam being condensed. The gases emerge from the top of the tower at about 180 F., the steam content thereof having been diminished to about by volume, of steamf The water is heated byA the gasesin its downward passage, and is.

discharged into the pond at the bottom of the tower, the water above the fixed level maintained by the pipe 20 being continu'- ously discharged by gravity into the blow pit and impinging against the stream of digester contents, in the event that it be desired to wash the pulp in the pit by the hot water delivered from the condenser tower. Otherwise, the eliuent liquor from the condenser may be cond-ucted to apoint external tothe blow pit. The pond water is discharged atabout 190-to 200 F., (preferably about 200 F.), and contains practically but little dissolved sulphur dioxide. If deliveredto the blow pit, it is heated by the stream fromi the digester and if at a temperature at which it absorbsSOz, the

absorbed gas is liberated. f The partially cooled gases from the rst tower are passed to the bottom ofthe cooling tower 35 through the pipe 27. This tower is initially at-'a temperature of 60 to 70- F. (depending on seasonable temperature of the water) so that the first portion of the gas to pass therethrough will leave the tower and be cooled to within a few degrees of the initial temperature of the mineral material. As the blow: progresses, the minerall material gradually heats so that thelast. portion of the gas may leave the toweri35 at about 110 to 120 F., the average temperature of the gas during an entir blowl being in the neighborhood of 100 The gases emerging from the top of the tower 35 pass through the pipe 41 to the absorp` tion tower 50, cold water or raw acid liquor being supplied thereto at the proper rate.l

thegas rising therethrough being absorbed f by the water or acid liquor, and the etiiuent being discharged'into the pipe 59.' When the blow is completed, the pipe line 41' between the towers 35 and 50 is closed by the Vvalve 43, and cold water is sprayed on thel surface material contained in the tower `35 until' it has assumed its initial low temperature of'about 60-70 F., the cooling water being run to waste' through the pipe 39. The system is then 'ready for the next blow.

By practicing the process, substantially all of the sulphur dioxide from the blow pit is recovered. Substantially no gas isl absorbed by the cooling water in the condenser tower, since the iow of the cooling water is so regulated as to effect its discharge from the condenser tower, at a temperature approximating 200y F. The seeond cooling of the gases. and vapor is indirectly effected by the water employed in cooling the interstitial material in the second tower during the time' interval between lblows, but, since itv does not come 'in direct contact with the gases, no precautionary measures are necessary for its regulation. The cooled sulphur dioxidel gas is finally recovered in whole or in part in the absorption tower, depending on the character of the liquid delivered thereto.

-said gases andV steam counter-current in direct contact with relatively cold water so as to condense .only a portion of the steam and partiallycool the gases and further cooling said gases without absorption and condensing out another portion of thesteam by passing them in contact with relatively cold inert interstitial material. 2'. A process for the recovery of sulphur dioxide from gases and steam liberated in the blow pit during theblowing' operation of a sulphite charge, which comprises passing sai gases an steamI in direct contact lwith relatively cold water trickling down a confined mass of surface material so as to condense only a portion of the steam andn to partially cool the gas, and discharging the cooling water and condensate at a temperature at which sulphur dioxide gas is practicall insoluble therein, land further cool-4 ing t e gases and condensing out a further portion of the steam bypassing the samev 1.30*

through a mass of relatively cold interj/ stitial material substantially free from extraneous water, and subsequently recovering the sulphur dioxide. 4

3. A process for the recovery of sulphur dioxide from gases and steam liberated in the blow pit during the successive blowing operations of a sulphite digester, which comprises passing said gases from each blow through relatively cold inert interstitial material without material absorption, and coolin said inert material substantially to its original temperature between successive blows.

4. A process for the recovery of sulphur dioxide from gases and vapor present in the blow pit-during the blowing operation of a sulphite digester, which comprises passing said' gases and lvapor counter-current to water tricklingdown inert interstitial material, thereby condensing only a portion of the vapor and partially cooling the gases, and further cooling said partially cooled gases by passing them through a tower containing relatively cold inert interstitial material without material absorption, and then recovering the cooled sulphur dioxide by passing the same into contact with an absorbent liquid and a 'surface material.

5. That step in the process of recovery of sulphur dioxide from gases liberated in the blow pit during the blowing operation of asulphite charge, which comprises partially cooling said gases by passing them through relatively cold interstitial material without absorption.

6. A process for the recovery of sulphur dioxide from gases liberated in the blow pit during the blowing operation of a sulphite digester, which comprises partially cooling said gases by passing them in direct contact with water trickling through a mass of interstitial material, and further cooling said gases by passing them through a second mass of cold inert interstitial material free from extraneous water.

7. A process for the recovery of sulphur dioxide from the steam and gases liberated terial, recovering sulphur dioxide gas ab-v -Sorbed by said cooling water by discharging 1t so that it impinges upon the digester contents as they are being blown, and further cooling the partially cooled gases and condensing'a further portion of the steam c0111` tained therein by passing through a second mass of cold inert surface material free from extraneous water.

8. A process for the recovery of sulphur dioxide from gases and vapor liberated in ythe blow pit during a sulphite blow, which comprises passing said gases in Contact with a relatively cold liquid medium, thereby partially coollng the gases yand condensing .only a portion of the vapor contained th( rein, passing said partially cooled gasesand vapor in direct contact with a relatively cold solid medium substantially free from extraneous water .to further cool -the same,

and recovering the sulphur dioxide contained in said cooled gases.

In testimony whereof we have affixed oury signatures.

GEORGE A. RICHTER. WALLACE B. VAN ARsDEL. 

